1. Field of the Invention
This invention relates generally to magnetic head servo control systems and, more particularly, to disk drive position control systems that determine the location of a head relative to disk tracks.
2. Description of the Related Art
In a conventional computer data storage system having a rotating storage medium, such as a magnetic or magneto-optical disk system, data is stored in a series of concentric or spiral tracks across the surface of the disk. A magnetic disk for example, can comprise a disk substrate having a surface on which a magnetic material is deposited. The digital data stored on a disk is represented as a series of variations in magnetic orientation of the disk magnetic material. The variations in magnetic orientation, generally comprising reversals of magnetic flux, represent binary digits of ones and zeroes that in turn represent data. The binary digits must be read from and recorded onto the disk surface in close proximity to the disk. That is, a read/write head can produce and detect variations in magnetic orientation of the magnetic material as the disk rotates relative to the head.
Conventionally, the read/write head is mounted on a disk arm that is moved across the disk by a servo. A disk drive servo control system controls movement of the disk arm across the surface of the disk to move the read/write head from data track to data track and, once over a selected track, to maintain the head in a path over the centerline of the selected track. Maintaining the head centered over a track facilitates accurate reading and recording of data in the track. Positioning read/write heads is one of the most critical aspects of recording and retrieving data in disk storage systems. With the very high track density of current disk drives, even the smallest head positioning error can potentially cause a loss of data that a disk drive customer wants to record or read. Accordingly, a great deal of effort is devoted to servo systems.
A servo control system generally maintains a read/write head in a position centered over a track by reading servo information recorded onto the disk surface. The servo information comprises a position-encoded servo pattern of high frequency magnetic flux transitions, generally flux reversals, that are pre-recorded in disk servo tracks. The flux transitions are recorded as periodic servo pattern bursts formed as parallel radial stripes in the servo tracks. When the read/write head passes over the servo pattern flux transitions, the head generates an analog signal whose repeating cyclic variations can be demodulated and decoded to indicate the position of the head over the disk. The position indicating information can be used to produce a corrective signal that is referred to as a position error sensing (PES) signal. The PES signal indicates which direction the head should be moved to remain centered over a track and properly read data.
There are a variety of methods for providing servo track information to a disk servo control system. In a method referred to as the dedicated servo method, the entire surface of one side of a disk is pre-recorded with servo track information. A servo head is positioned over the dedicated servo disk surface in a fixed relationship relative to data read/write heads positioned over one or more other data disk surfaces. The position of the servo head relative to the dedicated disk surface is used to indicate the position of the multiple data read/write heads relative to their respective disk surfaces. The dedicated servo method is most often used with multiple disk systems in which a servo head of a single dedicated servo disk surface controls movement of corresponding data read/write heads of a multiple platter disk drive.
Another method of providing servo track information is known as the sector servo method. In the sector servo method, each disk surface includes servo track information and binary data recorded in concentric or spiral tracks. The tracks on a sector servo disk surface are partitioned by radial sectors having a short servo track information area followed by a data area. The servo track information area typically includes a sector marker, track identification data, and a servo burst pattern. The sector marker indicates to the data read/write head that servo information immediately follows in the track. The servo read head is typically the same head used for reading data.
In both the dedicated servo and sector servo types of systems, the PES signal is used to generate a corrective input signal that is applied to the read/write head positioning servo. The remaining description assumes the sector servo system, but the manner in which the servo control system could be applied to a dedicated servo system will be readily apparent to those skilled in the art.
FIG. 1 is a representation of servo track information pre-recorded into a track 20 of a conventional disk 22 for an exemplary servo sector and data field. An initial field in the track comprises a synchronization field 24, such as for automatic gain control (AGC) or similar signal detecting purposes. The next field in the track is a track identification field 26, typically comprising a digitally encoded gray code. Next is a PES pattern field 28, generally containing a servo pattern burst, as described above. The next field in the track is a customer data synchronization field 30 for permitting read circuitry to adjust to the data amplitude and frequency, which may differ from those of the servo information. The data synchronization field 30 is followed by a customer data field 32.
FIG. 2 is a representation of a conventional disk drive quad-burst PES pattern in which magnetic transitions are recorded on the disk surface in bursts labeled as A, B, C, and D. The servo pattern bursts move relative to a disk drive magnetic head (not illustrated) from right to left. The disk data tracks and half-track positions arc indicated by the track numbers along the left side of the FIG. 2 drawing. The portion of the disk 22 shown in FIG. 2 extends approximately from track N-1.0 toward the inner diameter of the disk to half-track N+2.5 toward the outer diameter. Those skilled in the art will appreciate that position information is decoded by demodulating the signal generated by the head passing over the PES burst patterns to form a signal P based on: EQU P=A-C
and to form a quadrature signal Q based on: EQU Q=B-D.
The signals P and Q are quadrature signals because they are cyclic and arc out of phase by 90 degrees (one-quarter phase). The magnetic transitions that comprise the PES pattern are represented in FIG. 2 by vertical bars. The letter within each group of bars represents the PES burst recorded therein. One burst is distinguished from another by relative position in a track and relative position to the other bursts. Thus, for a read head that can detect magnetic transitions from more than one track at a time, the signal P should be zero when tracking exactly along the centerline of track N, because the head will detect equal amounts of magnetic field from the A and C servo bursts. A similar situation exists for tracks N+1, N+2. and so forth for tracks that are an even number multiple of half-tracks from N. For the half track position N+0.5, the signal Q should be zero when tracking exactly along the N+0.5 half-track line, because the head will detect equal amounts of field from the B and D servo bursts. The signal Q should be zero also for half-track positions N+1.5, N+2.5, and so forth.
The signal processing to which the servo readback signal is subjected includes demodulation and decoding of servo information and also includes automatic gain adjustment, to ensure that the readback signal is of relatively constant amplitude regardless of where the read/write head is relative to a track centerline or relative to the surface of the disk. The signal processing circuitry of the disk position control system includes a variable gain amplifier to perform automatic gain control (AGC). The AGC function is performed using an AGC data field contained in each servo sector. When the read/write head is over the AGC data field, the magnitude of the signal produced from the data field is examined and the gain of the variable gain amplifier is adjusted to provide a predetermined constant amplitude signal. In this way, processing gain variation for the associated servo demodulation circuitry can be eliminated, at least for readback signals of servo pattern bursts recorded at the frequency of the AGC data field pattern. Accordingly, most AGC data fields are recorded at the same frequency as the servo pattern bursts.
Some disk storage systems utilize dual frequency servo patterns. In a disk storage system with a dual frequency servo pattern, some of the servo bursts are recorded at a first frequency and the remaining servo bursts are recorded at a second frequency. The primary advantage claimed for dual frequency servo patterns is that they require less disk surface area. Nevertheless, such servo patterns typically do not provide performance that is as good as quad burst patterns in providing half-track position information.
To properly determine head position in a dual frequency disk storage system, it is typically necessary to provide a separate signal processing circuit for each servo pattern frequency. The signal processing functions performed for each circuit include filtering so the individual components of the quadrature signals can be distinguished for demodulation. In a single frequency system, the A and C servo bursts are distinguished in time, that is, by circumferential offset. In a dual frequency system, A and C are read at the same time and are distinguished via signal filter processing. The read/write head continuously receives the readback signal, and the filter processing determines which spectral portion of the received readback signal is part of which servo burst. Thus, processing circuits for each frequency receive the readback signal and produce the original bursts. This presents a problem because no single AGC pattern can be used for automatic gain control calibration for both frequencies. Therefore, the gain in the respective signal processing circuits for the two servo pattern frequencies cannot be automatically adjusted using the AGC field on the disk. More complicated systems must be used.
It is important for the readback signal processing of each frequency of a dual frequency system to be carefully gain-controlled. If the gains are oft, the centerline of the track following operation will be incorrect. The zero value of the PES, which usually corresponds to the track center, will instead be offset from the track center. Adjusting the signal gain during readback is important because the relative gain of the readback signal can change with disk fly height. The disk fly height is a measure of how high above the disk surface the read head is traveling. With a single frequency PES servo pattern, any fly height variation in the readback signal can be calibrated out with AGC processing and the variable gain amplifier described above. With a dual frequency servo pattern, it is impossible to eliminate all calibration error using conventional AGC processing. At most, only one of the servo pattern frequencies can be calibrated using the AGC field, because both P and Q components of the readback signal are being received at the same time, and at most one of the PES servo pattern frequencies is equal to the frequency of the AGC field. Thus, it is very difficult to automatically correct the gain for the readback signal produced from demodulating both of the servo pattern bursts.
From the discussion above, it should be apparent that there is a need for a dual frequency disk drive storage system that can perform accurate and reliable automatic gain control during readback for both servo frequencies, and that can respond to variations in disk fly height for proper gain calibration in the signal demodulator. The present invention fulfills this need.